Ok so I'm new to AG brewing, I did a brew last week and after I'd mashed for 90mins I started to sparge. As the wort started to run I did a hydrometer test as I've read. Now with a Hydrometer Temperature Correction Calculator I hit 1070, from a recipe that should have been 1045. What the hell happened there.
This is the 2nd time that I've hit well over the target for a recipe, am I doing something wrong? I've worked out that if it finishes out where I hope it will, it will be about 7% when the recipe stated 4.5%

The gravity at the beginning of the runnings, when it appears you took your reading, will be higher than towards the end. The first runnings are essentially the liquid that contains all the sugar from the grain, while the later sparge will be just the sugar rinsed from the grain.

If you did measure the initial runnings, your OG will be significantly lower.

The other potential issue is the recipe itself. Perhaps it was formulated for a larger volume or a lower efficiency than your system delivers. The recipes in Brew-Your-Own and Brewing classic Styles are both designed for a rather low efficiency and BCS targets 6 gallons of final beer.

If you post the recipe, I can take a look and see if the recipe makes sense.

To get the gravity of your wort, collect all the wort into your kettle, measuring the volume, and stir thoroughly. I find it difficult to stir thoroughly enough to get the wort mixed enough to get an accurate measurement, so I wait until the wort comes to a boil, which mixes the wort well. Then, I cool a sample and take the gravity.

The gravity of the first runnings will be higher than the final sparge water, so that will not tell you the gravity of the total wort. For the mash thickness you used, the first runnings should be close to 1.070, so it does look like you got very good conversion of the starch to sugar.

For reference, here are some numbers for that recipe: at 75% mash efficiency, which is a commonly used efficiency among homebrewers, the OG would be ~1.045 for your final volume. An well designed system with near 100% conversion would be expected to deliver roughly 85% efficiency from that amount of grain, which would give an OG of ~1.051 (from your numbers, I suspect your beer was close to this). Even if you managed to get 100% of the sugar out of the grain, you wouldn't get an OG over 1.060.

Thanks for that. Slothrob, looks like I didn't do to bad after all, the yeast fired up without much lag time, in fact by the following morning it was going like a train. Calmed down a little now, I'll see what the SG is like on Friday as I will be off work, it's a bank holiday this weekend 4 days off over here